1. Field of the Invention
This invention relates generally to computer/television video systems, and specifically to providing copy protection to computer video signals.
2. Description of the Related Art
The advent of Digital Video Disc (DVD) players has created excitement in the entertainment and consumer electronics industry. DVD offers clearer pictures, clearer audio, and more capabilities than most other video playback devices. In addition to use as a playback mechanism in connection with television, DVD players can also be coupled to a personal computer (PC) as a Read-Only type data storage device in the form of a DVD-ROM (Read Only Memory). With a DVD player installed in a computer, the computer can play high-quality video on the computer monitor. Typically, conventional computers allow any data that is accessed by the computer to be copied to another location. Thus copyrighted audiovisual material stored on aa DVD-ROM such as movies, songs, shows, etc., may be freely copied on a PC equipped with a DVD-ROM player. As can be seen, with this advancement in technology provided by DVD comes a price: the possibility of pirated copies of copyrighted material.
Currently, copyrighted material played on conventional analog VHS recorders may be protected from pirated copying by technology available from the Macrovision Corporation. VHS recorders typically employ an Automatic Gain Control (AGC) circuit to apply gain to boost an incoming signal""s level to a level for acceptable viewing. The VHS recorder determines the amount of gain to add by finding the maximum picture amplitude of the incoming signal. The maximum picture amplitude is then used as a baseline to determine how much gain the signal needs. The Macrovision technology uses electronic pulses to fool the VHS recorder into believing that picture information is of different characteristics from the real image content.
A DVD player coupled to a PC presents a different problem. Many PCs are now capable of converting a digitally encoded signal into an analog television signal for transmission to a television or a VHS recorder thereby making it possible to make a VHS tape recording of a DVD movie played by a computer""s DVD player. Typically, such PCs encode a DVD signal for a standard television format, such as NTSC. A VHS recorder can be coupled to a port on the PC which provides an analog television format (e.g. NTSC) signal. Ordinarily, this would allow easy copying (legal or illegal) of the contents of a DVD type disc. However, the Macrovision process described above is now typically added to the signal being transmitted to the television, thus providing the same effective protection as described above.
The addition of the Macrovision process to PCs does not, however, eliminate another easily accessible means of copying. PCs which have a port for TV type signals typically also have a port (monitor port) to which a conventional computer monitor can be coupled. The monitor port typically is used to transmit an image which is the same or similar (but formatted differently) to that transmitted via the TV port. A would-be pirate can copy the DVD signal directly from the PC""s VGA port by using a conventional VGA-to-TV (NTSC or PAL) converter. The Macrovision process cannot be added to the RGB signal prior to reaching the VGA port because the pseudo sync pulses would impair the computer monitor""s ability to display RGB signals.
The other problem with current copyright protection schemes are their ability to be easily defeated by simple reverse engineering. Thus, there is a need for a copyright protection mechanism that produces a signal that can be accurately displayed on a computer monitor, but at the same time be of extremely poor image quality if converted to a television-compatible signal and copied using a VHS-type recorder. There is also a need for such a mechanism that is not easily defeated by reverse-engineering.
In a principal aspect the present invention provides copyright protection for the now-unprotected computer monitor port of a computer. Embodiments employing the principles of the present invention accomplish this goal in two ways: (i) delaying the synchronizing signals by a time variable amount and (ii) pulse generation during non-active video. Although either method alone may deter illicit copying, together the two systems advantageously produce a signal that is of sufficiently low picture quality to deter illicit copying and ""brand""the image as an image that was intended to be protected.
In accordance with the present invention, in certain embodiments a horizontal synchronizing signal sent to the monitor is delayed or advanced in time. In this embodiment, a fixed offset, or delay, is selected. In addition, a pseudorandom delay is introduced in conjunction with the fixed offset. Either the pseudorandom delay or the fixed delay is sent to a variable delay generator, which delays the horizontal synchronizing signal by the amount of the delay factor. The delayed horizontal synchronizing signal is then sent to the computer monitor, which uses the delayed synchronizing signal to produce its display in conjunction with data signals. Thus, if the data signal or the synchronizing signals are intercepted by a VGA to TV converter, that converter (or any downstream device) is unable to lock onto the correct frequency in order to reproduce the image properly.
Another embodiment of the present invention causes the generation of pulse bars that also renders a signal unfit for taping purposes. In accordance with this embodiment of the invention, horizontal pulse bars are generated to replace the portion of the video signal containing no picture information after vertical picture contraction has occurred. The amplitude of the input signal 212 is reduced, and the amplitude of the horizontal pulse bars is increased to be greater than the amplitude of the input signal. The horizontal pulse bars are then combined with the attenuated signal. When a VHS recorder detects the amplitude of the modified signal, it lowers the amount of gain that it adds to the modified signal because of the high amplitude of the added pulses. As the data content of the signal is at a much lower amplitude than the added pulses due to the attenuation, the viewable picture after taping is extremely dim and of poor quality. The pulse characteristics may be varied depending on user preferences by way of programmable registers.
As a computer monitor is designed to read quickly and follow horizontal synchronizing signal disturbances, the modifications to the signal above would have no detrimental effect when the signal is displayed on a computer monitor. Additionally, attenuating the amplitude of the input signal has no effect on the display of the signal due to the gamma characteristics of the computer monitor. These and other features and advantages may be better understood with reference to following specification.